1. Field of the Invention
This invention relates to an injection unit for plasticizing a feed resin and injecting it into a mold in an injection molding machine for plastics, and especially to an injection unit suited for the injection molding of feed resins, such as powdery-filler-added resins and long-fiber-filled resins.
2. Description of the Related Art
FIG. 12 is a cross-sectional view schematically illustrating the overall construction of a conventional injection unit in an injection molding machine. As is depicted in FIG. 12, the injection unit is constructed of a hopper 102, an injection cylinder 103, a plasticizing screw 104, a heater 105, a nozzle 106, a bearing box 107, a motor 108 and a hydraulic cylinder 109.
Here, the hopper 102 serves to store therein a feed resin 101 which is to be fed into the injection cylinder 103. The injection cylinder 103 internally holds the plasticizing screw 104 so that the plasticizing screw 104 is rotatable and also slidable in a forward/rearward direction (an axial direction, i.e., a horizontal direction as viewed in FIG. 12), and at a free end (forward end) of the injection cylinder, a nozzle 106 is formed for injecting the molten and plasticized feed resin (molten resin) into an unillustrated mold.
The plasticizing screw 104 is provided on an outer periphery thereof with a screw flight 104a for feeding the feed resin 101 forward and, at a basal end portion thereof, is rotatably supported by the bearing box 107 and is connected to the motor 108. The plasticizing screw 104 is driven by the motor 108 to feed the feed resin 101 forward (in the leftward direction in FIG. 12) from the hopper 102 while plasticizing the resin.
On a side of a free end (on a side of a free end forward of the screw flight 104a) of the plasticizing screw 104, a seat 115 and a screw tip 117 are formed and a check ring 116 is arranged.
In general, it is necessary for the injection unit to provide on the side of the free end of the plasticizing screw 104 with a function as a check valve that allows a molten resin 114 to pass to a forward part of the plasticizing screw 104 during a plasticizing step and, upon injection, prevents a reverse flow of the molten resin 114 to inject the molten resin 114 in a quantity corresponding to the volume of a product to be molded. In the conventional injection unit shown in FIG. 12, this function is exhibited by the check ring 116 and the seat 115.
Specifically, the check ring 116 is arranged on the outer periphery of the screw tip 117 in such a way that a space 118 is left between the check ring 116 and the screw tip 117, the check ring 116 is slidable relative to the screw tip 117 and, upon injection, the check ring 116 is brought at a rearward end thereof into contact with the seat 115 formed on the outer periphery of the plasticizing screw 104 so that the resin is prevented from flowing backward.
In addition, the check ring 116 is fitted so that the check ring 116 is in contact with an inner peripheral wall of the injection cylinder 103 and is rotatable and slidable along the inner peripheral wall. An outer diameter of the seat 115 is set smaller than an inner diameter of the injection cylinder 103 so that a clearance 119 is formed between an outer periphery of the seat 115 and the inner peripheral wall of the injection cylinder 103. The seat 115 and the check ring 116 are arranged so that they rotate together with the screw tip 117 during rotation of the plasticizing screw 104. During a plasticizing step, the above-mentioned space 118 and clearance 119, as will be described subsequently herein, also function as flow passages through which the molten and plasticized resin passes.
The heater 105 is disposed on the outer periphery of the injection cylinder 103 to heat the resin which is being rotated by the plasticizing screw 104 (the screw flight 104a) within the injection cylinder 103. The hydraulic cylinder 109 is formed integrally with the injection cylinder 103, and causes the plasticizing screw 104, the bearing box 107 and the motor 108 to move in the axial direction of the screw (the forward/rearward direction) relative to the injection cylinder 103.
Within the hydraulic cylinder 109, a hydraulic piston 110 which is fixed to the bearing box 107 via a rod 110a is held slidably in the forward/rearward direction. By supplying hydraulic fluid to a fluid compartment 109a on a rearward side of the hydraulic piston 110 through a port 111 from an unillustrated hydraulic fluid supplying system, the plasticizing screw 104 is caused to advance relative to the injection cylinder 103. By supplying hydraulic fluid to a fluid compartment 109b on a forward side of the hydraulic piston 110 through a port 112 from the unillustrated hydraulic fluid supplying system, on the other hand, the plasticizing screw 104 is caused to retreat relative to the injection cylinder 103.
In the conventional injection unit constructed as described above, the feed resin 101 is supplied from the hopper 102 to the outer periphery of the plasticizing screw 104 (screw flight 104a) within the injection cylinder 103, is molten and plasticized under heating by the heater 105 and also under rotation by the plasticizing screw 104, is fed to a point forward of the plasticizing screw 104. At this time, the feed resin is allowed to pass through the clearance 119 and space 118 as indicated by streamlines g, and is then stored as the molten resin 114 on a side forward of the screw tip 117. During such a plasticizing step, a space is maintained between the check ring 116 and the seat 115 as shown in FIG. 12.
By a forward movement of the plasticizing screw 104, the molten resin 114 is injected into the unillustrated mold through the nozzle 6, and is then cooled and solidified into a molded product. During such an injection step, hydraulic fluid is supplied to the fluid compartment 109a in the hydraulic cylinder 109 through the port 111. The hydraulic piston 110 is hence caused to advance relative to the injection cylinder 103, in other words, the plasticizing screw 104 is thus caused to advance relative to the injection cylinder 103, whereby the molten resin 114 is injected through the nozzle 106.
At this time, the check ring 116 is pushed rearward by reaction force of the molten resin 114, so that the check ring 116 is brought into close contact with the seat 115 to eliminate the space between the check ring 116 and the seat 115. It is therefore possible to prevent the molten resin 114 from flowing backward toward the screw flight 104a of the plasticizing screw 104 so that, upon injection, the molten resin 114 can be injected in a quantity corresponding to the volume of a product to be molded.
According to the above-described conventional injection unit, however, the plasticizing resin is fed to a point forward of the screw tip 117 through a narrow passage (the space 118) inside the check ring 116, which functions as a check valve, as indicated by the streamlines g when fed to the forward part of the plasticizing screw 104. The resin which is flowing through the flow passage therefore encounters significant flow resistance. Problems such as those to be described next may therefore arise when a resin filled with a powdery filler, such as woodflour or coal ash or a resin filled with long fibers is used.
In the case of the resin filled with the powdery filler, the resin-feeding ability of the plasticizing screw 104 is lowered due to the inclusion of the powdery filler compared with ordinary pellet-like resins, so that the plasticizing ability may be substantially reduced. Further, the narrow flow passage inside the check ring 116 may be clogged by the powdery filler, thereby making it impossible to continue the molding.
In the case of the resin filled with the long fibers, the fibers may be cut into shorter fibers due to shearing action within a screw channel formed by the screw flight 104a on the outer periphery of the plasticizing screw 104 and also due to flow resistance within the narrow flow passage inside the check ring 116. The strength-improving effect of the long fibers for the molded product is hence reduced.
Further, use of a feed resin containing a component, which evaporates into gas upon plasticizing like pyrolignous acid in woodflour, may lead to occurrence of a defect, such as bubbles or burn marks, in the molded product (due to compression by the gas). With a view to avoiding the occurrence of gas in such a plasticizing step, it may be contemplated to preheat and dry the molding feed resin at a high temperature. This preheating, however, requires extra time and expense and are therefore not preferred.
According to the conventional unit shown in FIG. 12, the feed resin 101 is plasticizing while being fed forward by rotation of the plasticizing screw 104. Depending on the type of the resin, there is a potential problem that, before the feed resin 101 is fully molten, it may be fed into the molten resin 114 and may then be injected in a still unmolten form. It is therefore difficult to mold a wide variety of resins by using the same plasticizing screw 104. Screws of different shapes must hence be employed for resins of different types, such as general-purpose resins (polypropylene, polystyrene, and the like), high-viscosity resins and vinyl chloride resin, respectively.
The plasticizing of the feed resin 101 must be completed before the resin is fed to the point forward of the plasticizing screw 104 (screw tip 117), and the molten resin 114 must be accumulated in a predetermined quantity at the point forward of the plasticizing screw 104. Accordingly, a screw length about 20 times as much as a screw diameter is needed, and the injection unit has a longer machine length. This has led to a problem that an extra installation space is required accordingly.
Incidentally, JP Kokai 4-163015, for example, discloses a technique that a discrete plasticizing unit is attached to an injection cylinder and a molten resin plasticized by the plasticizing unit is fed to a point forward of an injection plunger within an injection cylinder. This technique has made it possible to provide an injection unit with a shorter machine length. In the plasticizing unit, a screw provided with a continuous flight is used and performs substantially the same function as conventional plasticizing screws. Namely, whenever the screw of the plasticizing unit rotates, the resin is fed toward the injection cylinder. Therefore, a time until the resin is fed into the injection cylinder along an outer periphery of the screw is a plasticizing time, which cannot be set as desired. In the case of a short screw length, the plasticizing time is not sufficient so that the melting of the resin is insufficient. The unmolten resin may be fed into the injection cylinder, and may be injected as is.